To evaluate the safety and effectiveness of Ara-C and the antiviral medications AZT, ddI, ddC for the treatment of PML. Participants will be randomized to one of the following 3 arms: 1) antiviral medication only, 2) antiviral medication plus IV Ara-C, 3) antiviral medication plus intrathecal Ara-C. Participants will have brain biopsy proven PML. Those participants who are in either of the groups receiving Ara-C will receive their Ara-C treatment in the GCRC.